Is your boiler pressure gauge dropping into the red zone? A boiler that keeps losing pressure is one of the most common heating problems in the UK — and in most cases, it’s caused by a faulty part that you can replace yourself or with the help of a Gas Safe engineer.
At Boiler Spares Ltd, we stock the genuine and compatible parts you need to fix pressure problems fast. Here’s a complete guide to the causes, the fixes, and the parts you’ll need.
Why Is My Boiler Losing Pressure?
Boilers need stable pressure (usually between 1 and 1.5 bar) to circulate hot water around your home. When the pressure drops, the boiler will often lock out and display an error code like E119 (on Baxi boilers) or simply fail to fire.
The most common causes are:
1. A Leak in the Heating System
Before you buy any parts, check for visible leaks. Look under radiators, around pipe joints, and under the boiler itself. Even a small leak can cause pressure to drop over days or weeks.
Quick test: Top up the pressure via the filling loop and watch the gauge. If it drops within 24 hours, you likely have a leak.
Fix: Fix the leak first. If the pressure still drops after the leak is sealed, move on to the parts below.
2. Faulty Pressure Sensor (Most Common Part Failure)
The pressure sensor (also called a pressure switch or transducer) tells the boiler’s PCB what the system pressure is. When it fails, the boiler either misreads the pressure or thinks it’s too low.
Signs of a faulty pressure sensor:
- Pressure gauge reads normally but boiler still locks out
- Intermittent pressure errors
- Boiler works after re-pressurising but fails again quickly
Fix: Replace the pressure sensor. These are inexpensive and easy to fit. We stock pressure sensors for Baxi, Worcester, Vaillant, Ideal, and Glow-worm boilers.
3. Failed Expansion Vessel
The expansion vessel absorbs the natural expansion of water as it heats up. If the internal diaphragm splits or the vessel loses its air charge, the pressure has nowhere to go — so it rises when heating, then drops rapidly when the system cools.
Signs of a failed expansion vessel:
- Pressure rises above 2.5 bar when heating is on
- Pressure drops below 0.5 bar when the system cools
- Pressure relief valve (PRV) is constantly dripping
Fix: Recharge the expansion vessel with a pump if possible. If the diaphragm has split, you’ll need a replacement expansion vessel. We stock flat and round vessels in all common sizes.
4. Dripping Pressure Relief Valve (PRV)
The safety valve (or pressure relief valve) opens to release water if the pressure gets too high. If the expansion vessel has failed, the PRV will open regularly — and once a PRV starts dripping, it often won’t reseal properly.
Signs of a faulty PRV:
- Water dripping from the copper pipe outside
- Pressure drops even though the system isn’t leaking
- PRV has been activated multiple times
Fix: Replace the safety valve / PRV. Always check the expansion vessel at the same time, as a failed vessel is usually what caused the PRV to open in the first place.
5. Filling Loop Left Open
Sometimes the simplest explanation is the right one. If the filling loop (the braided hose under the boiler) is left open, water can slowly enter the system and then escape through the PRV.
Fix: Close the filling loop valves tightly after re-pressurising.
Quick Diagnosis Table
Table
| Symptom | Likely Cause | Part Needed |
| Pressure drops over 1–2 weeks | Small leak or faulty sensor | Pressure sensor |
| Pressure shoots up when heating, then drops | Failed expansion vessel | Expansion vessel |
| Water dripping from outside pipe | PRV opened due to high pressure | Safety valve + expansion vessel |
| Pressure drops within 24 hours | Significant leak or filling loop open | Fix leak / close loop |
Can I Fix Pressure Problems Myself?
- Re-pressurising: Yes — any homeowner can do this
- Replacing a pressure sensor: Yes — it’s a simple electrical connection (isolate power first)
- Replacing a safety valve: Yes — but isolate water and power first
- Replacing an expansion vessel: Possible for confident DIYers, but it’s heavy and requires draining the system. A heating engineer may be better
- Any gas work: No — always use a Gas Safe registered engineer
Shop Boiler Pressure Parts
At Boiler Spares Ltd, we stock everything you need to fix boiler pressure problems:
All with fast UK delivery from Huddersfield. Order before 3pm for same-day dispatch on most items.
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